Future wireless and/or cellular systems are expected to either require increased coverage, support higher data rates or a combination of both. In addition, the cost aspect of building and maintaining the system is expected to become of greater importance in the future.
One aspect is rethinking the topology used in existing systems, as there has been little change of topology over the three generations of cellular networks. For instance, it is well known that multihopping, being an example of another communication topology, offers possibilities of significantly reduced path loss between communicating (relay) entities, which may benefit the user. In the following, another type of topology will be discussed that considers two-hop relaying combined with aspects of advanced antenna systems. This is a research area, yet in its infancy, that employs cooperation among multiple stations as a common denominator. In recent research literature, it goes under several names, such as cooperative diversity, cooperative coding, virtual antenna arrays, etc. A good general overview over cooperative communication schemes is given in [1]. The general benefits of cooperation between stations in wireless communication can be summarized as higher data rates, reduced outage (due to some forms of diversity), increased battery life and extended coverage (e.g. for cellular systems).
When routing is applied in a wireless communication network, such a network is often denoted a multihop network. In a multihop network, nodes or stations out of reach from each other can benefit from intermediate located nodes that can forward their messages from the source node towards the destination node. Traditionally, multihop networks have been associated with so called ad hoc networks, where nodes are mostly mobile and no central coordinating infrastructure exists. However, the idea of multihop networking can also be applied when nodes are fixed and/or a central coordinating infrastructure exists. One such scenario targets rural area Internet access and uses fixed nodes attached to the top of house roofs, lamp posts and so forth.
In a multi-hop scenario, information may be transmitted over multiple hops between source and destination instead of directly in a single hop. In general, the multi-hop approach offers several advantages such as lower power consumption and higher information throughput compared to a direct one-hop approach. In a multi-hop network, nodes out of reach from each other can benefit from intermediately located nodes that can forward their messages from the source towards the destination.
One method to provide enhanced coverage and data rate is to use so called 2-hop relaying, which could be viewed as a degenerate case of multihop employing only two hops. The benefit consequently stems from splitting a long transmission distance into two roughly equidistant hops to allow increased data rate on each link as well as the end to end (ETE) total rate.
In one specific form of 2-hop relaying, a transmitter sends a signal to a relay that after receiving the message forwards it to a receiver. At the same time, the signal is sent directly to the receiver. The receiver could then enhance the quality of the signal, by combining the, at two different time instances, received first direct signal and the second relayed signal. Enhanced quality means increased average and less variance of the data rate.
A particular form of 2-hop relaying is often denoted cooperative relaying (CR). (It should be noted that also the previous scheme with only one relay node is sometimes also, although questionably, denoted CR). The basic idea of this particular form of CR is to use various aspects of “cooperation” among relay nodes. For example, a signal sent by a transmitting node may first be received by multiple relays and subsequently and concurrently forwarded and received by a receiving node.
In CR, the relays are generally allowed to perform various signal processing or coding tasks that in various ways improve the overall communication performance. The benefits of the mechanisms that are exploited in CR can broadly be divided into, diversity gain, beam-forming gain, and spatial multiplexing gain. Also, here, the receiver could enhance the quality of the signal by combining the first direct signal and the second relayed signals. An overview of CR schemes is given in [1].